1. Technical Field
This invention relates to a position detecting device that is used together with a position indicator to detect a position indicated by the position indicator. Furthermore, this invention relates to a display apparatus including a position detecting device, and a portable apparatus including a position detecting device.
2. Description of the Related Art
The position detecting device to detect a position indicated by a position indicator, such as a finger or a pen, is well known. As position detecting systems used in the position detecting devices, various kinds of systems such as the resistive film system, the electromagnetic induction system, and the capacitive system are known. An example of the position detecting device that uses the electromagnetic induction system is described in Japanese Patent Laid-open No. 2009-3796 (referred to as Patent Document 1).
Specifically, in the position detecting device described in Patent Document 1, as shown in FIG. 10, a sensor substrate 3 and a magnetic path plate 4 are disposed between an upper case 1 and a lower case 2. The sensor substrate 3 has coils 5 in which an X-axis direction loop coil group (not shown) and a Y-axis direction loop coil group are disposed on both surfaces of the sensor substrate 3.
This position detecting device is used together with a position indicator 6. The position indicator 6 includes a resonant circuit composed of a coil 6L and a capacitor 6C.
In this position detecting device, each loop coil of the X-axis direction loop coil group and the Y-axis direction loop coil group of the coils 5 is selected by a selection circuit (not shown) according to a predetermined procedure. To the loop coil selected by the selection circuit, a predetermined alternating signal is supplied for a predetermined time. After the predetermined time has elapsed, the supply of the alternating signal to the selected loop coil is stopped and the loop coil is switched to the signal receiving state.
During the predetermined time when the alternating signal is supplied to the loop coil, an electromagnetic wave (alternating magnetic field) is generated from the loop coil and the alternating magnetic field is supplied to the resonant circuit of the position indicator 6. Thus, energy is stored in the resonant circuit.
After the supply of the alternating signal to the selected loop coil is stopped and the state is switched to the signal receiving state, an electromagnetic wave is transmitted from the position indicator 6 to the coils 5 based on the energy stored in the resonant circuit of the position indicator 6. Therefore, in the position detecting device, reception of the electromagnetic wave from the position indicator 6 can be sensed by monitoring the signal from the selected loop coil of the coils 5.
In the above-described manner, transmission and reception of the electromagnetic wave are performed between the coils 5 on the sensor substrate 3 and the position indicator 6. Furthermore, through selection of the loop coil by the selection circuit according to a predetermined procedure and transmission and reception of the electromagnetic wave between the loop coil and the position indicator 6, the position indicated by the position indicator 6 is determined.
Regarding the electromagnetic wave transmitted and received in the above-described manner, the magnetic path plate 4 forms a magnetic path for the alternating magnetic field generated by the coils 5. Accordingly, the magnetic path plate 4 prevents the spread of the generated magnetic flux, to thereby enhance the detection sensitivity of the position detecting device to the position indicator 6. Furthermore, the magnetic path plate 4 has a function to prevent an alternating magnetic field from radiating to the outside of the position detecting device and to prevent an electromagnetic wave from outside of the position detecting device from mixing in as noise with the electromagnetic wave transmitted and received in the above-described manner.
In other words, the magnetic path plate 4 is provided in order to effectively form a magnetic path for the alternating magnetic field. For this purpose, currently a material having high permeability, such as an amorphous metal, is used.
FIG. 11 shows a magnetic path formed when an amorphous metal is used as the material having high permeability for the magnetic path plate 4. In FIG. 11, the magnetic path plate 4 is composed of a magnetic substance of high permeability, such as an amorphous metal. The magnetic path plate 4 is composed of an amorphous metal whose permeability is as high as 10000 (H/m), for example, and therefore magnetic flux (alternating magnetic flux) generated due to the alternating signal supplied to the coils 5 effectively forms a magnetic path 4a in the magnetic path plate 4.
The amorphous metal has extremely low electrical resistance and therefore an eddy current is generated therein based on the magnetic flux applied to the magnetic path plate 4. The eddy current acts to cancel the applied magnetic field. However, even in view of the demerit associated with generation of the eddy current, the amorphous metal having high permeability exerts high performance as a magnetic path plate as a whole. Thus, an amorphous metal has been used to form a magnetic path plate for a position detecting device.